Simultaneous interpreting (SI) involves concurrent discourse processing operations in two languages, in real time and in an immediate context. Omission of information is a frequent phenomenon in this linguistic mediation task and it may vary from the complete loss of information to the deletion of selected items rendered as redundant or irrelevant. Some theoretical approaches ascribe omission to involuntary failures in the distribution of cognitive resources for discourse processing; while others claim that omission is a deliberate strategy of interpreters, aimed at adapting their speech to the information requirements of their audience and thus complying with norms of professional performance. This study explores the cognitive-discourse factors associated to omission of difficult segments in an English-Spanish SI and the circumstances in which such loss of information was more frequent. The results indicate that a number of cognitive and contextual factors interact to 1) cause the involuntary interruption of the interpretation process, with the consequent loss of information and 2) trigger the strategic decision to omit information. Some of these factors are lack of prior knowledge, problems in the decodification/comprehension of the source speech, application of interpreting norms, among others. These results are discussed in relation to the theoretical and empirical framework, as well as the implications of these findings to interpreter training and interpretation quality assessment.